A method is known to control a voice signal given as an input signal such that the voice signal is easy to listen. For example, for aged people, a voice recognition ability may be degraded due to a reduction in hearing ability or the like with aging. Therefore, it tends to become difficult for aged people to hear voices when a talker speaks at a high speech rate in a two-way voice communication using a portable communication terminal or the like. A simplest way to handle the above situation is that a talker speaks “slowly” and “clearly”, as disclosed, for example, in Tomono Miki et al., “Development of Radio and Television Receiver with Speech Rate Conversion Technology”, CASE#10-03, Institute of Innovation Research, Hitotsubashi University, April, 2010. In other words, it is effective that a talker speaks slowly word by word with a clear pause between words and between phrases. However, in two-way voice communications, it may be difficult to ask a talker, who usually speaks fast, to intentionally speak “slowly” and “clearly”. In view of the above situation, for example, Japanese Patent No. 4460580 discloses a technique in which voice segments of a received voice signal are detected and extended to improve audibility thereof, and furthermore, non-voice segments are shortened to reduce a delay caused by the extension of voice segments. More specifically, when an input signal is given, a voice segment, that is, an active speech segment and a non-voice segment, that is, a non-speech segment in the given input signal are detected, and voice samples included in the voice segment are repeated periodically thereby controlling the speech rate to be lowered without changing the speech pitch of a received voice and thus achieving an improvement in easiness of listening. Furthermore, by shortening a non-voice segment between voice segments, it is possible to minimize a delay caused by the extension of the voice segments so as to suppress sluggishness resulting from the extension of the voice segments thereby allowing the two-way voice communication to be natural.